1. Field of the Invention
The present invention relates to a drilling method for making holes through a workpiece while replacing a plurality of drills.
2. Related Art
FIG. 4 is a front view showing a vicinity of a spindle of a printed board drilling apparatus partially in section and FIG. 5 is a plan view showing a part of a table of the printed board drilling apparatus. As shown in FIG. 4, a spindle 10 rotatably holds a drill 15 at an edge thereof. A pressure foot 20 that fits with the edge part of the spindle 10 is supported so as to be movable in a Z-axis direction and is urged downward in the figure by air cylinders not shown. A plurality of printed boards 2a to be machined is laid upon an under plate 2b and is mounted on a surface 1a of the table 1 while being fixed together by two reference pins 3a and 3b (normally, the diameter of the reference pins 3a and 3b is equal). The printed board 2a to be machined and the under plate 2b will be referred to as the printed board 2 altogether hereinafter.
As shown in FIG. 5, holes (here, in a shape of cutout) 4 and 5 are formed on the surface side of the table 1. A square first clamp plate 7 is disposed by the hole 4 where a V-shaped groove 6 is formed at one side thereof. A rectangular second clamp plate 8 is disposed by the square hole 5. As shown in FIG. 4, the clamp plates 7 and 8 are supported by a linear guide unit 9 comprising a bearing 9a and a track 9b disposed at the bottom of the holes 4 and 5 so that their upper faces are leveled with the surface 1a of the table 1 and are movable in a Y-axis direction by driving means not shown.
A number of types of diameter of the holes made through the printed board vary, from several, to ten-odd types, and a number of the holes to be made is also large. Then, several tens to several hundreds of drills are disposed in the printed board drilling apparatus.
Next, machining steps of the conventional printed board drilling apparatus constructed, as described above, will be explained. First, the printed board 2 is mounted on the table 1 in a state in which the clamp plates 7 and 8 stay on the right side of the figure, as shown by solid lines in FIG. 5. Then, the clamp plates 7 and 8 are moved to the left side in the figure, so that the first reference pin 3a abuts against the groove 6, and the second reference pin 3b abuts against a right side face 8a. Then, the printed board 2 is positioned in the Y-axis direction as a line O, connecting centers of the two reference pins 3a and 3b, becomes parallel with an X-axis of the printed board 2 and is also positioned in the X-axis direction by the groove 6 as disclosed in Japanese Patent Laid-Open No. 2003-1594, for example.
The table 1 is then moved in the X-axis direction and the spindle 10 is moved in the Y-axis direction to align the drill 15 to part of the printed board 2 to be machined. Then, the spindle 10 is moved in the Z-axis direction so that the drill 15 cuts into the printed board 2 while pressing the printed board 2 by the pressure foot 20 to drill the printed board 2. When the printed board drilling apparatus receives, from a machining program, a command or instruction that instructs the apparatus to replace the tool due to the life of the tool, the printed board drilling apparatus replaces the drill 15 and continues the machining.
It is noted that while the printed boards are separated one by one after being drilled and patterns and others are formed thereon in post-processing, such as exposure and etching processes, the holes into which the reference pins have been inserted deform in separating the printed boards one by one. Then, in addition to the holes necessary as a product, reference holes for post-processing (referred to exposure master holes hereinafter) are drilled to that end.
By the way, an air spindle is adopted as the spindle in case of the printed board drilling apparatus and a number of revolutions is set at around 200,000 rpm in making a small hole of 0.3 mm or less, at around 60,000 rpm in making a hole of around 1.0 mm and at around 40,000 rpm in making a hole of 2.0 mm or more. In case of the air spindle, an axial line of the spindle, i.e., an axial line of the drill, is misaligned from designed position corresponding to the number of revolutions because exothermic value of the spindle changes and pressure of air to be supplied to the spindle is changed corresponding to the number of revolutions. However, the degree of misalignment of the axial line of the drill (hereinafter referred simply as “misalignment”) from the designed position corresponding to the number of revolutions of the spindle is intrinsic to the spindle. Accordingly, when a number of axes of the printed board drilling apparatus is one, i.e., when a number of spindles is one, it is possible to machine the printed board with excellent machining accuracy by detecting the misalignment of the spindle per number of revolutions in advance and by correcting that misalignment.
However, in order to increase machining efficiency, a multi-spindle printed board drilling apparatus capable of mounting a plurality of printed boards on one table and having a number of spindles equal with that of the printed boards is often adopted as the printed board drilling apparatus.
FIG. 6 is a plan view showing a surface of a printed board machined by a predetermined one spindle of a multi-spindle printed board drilling apparatus and FIG. 7 is a graph showing dispersion of machining positions of the exposure master hole and a hole of 0.2 mm in diameter machined by a bi-spindle printed board drilling apparatus. In FIG. 6, a range K surrounded by a broken line is a range in which the printed board, i.e., a product, is disposed and in which several ten thousands to several hundred thousands of holes of 0.1 mm in diameter and several thousands to several ten thousands of holes of 0.2 to 0.4 mm in diameter for example are made. Two holes C disposed on the outside of the range K are the exposure master holes which are made in a direction of a diagonal line in the figure.
In FIG. 7, Acx and Acy represent an average value of misalignment of the center of the exposure master hole, machined by a first spindle among the multiple spindles, with respect to the designed center and Amx and Amy represent a center value of distribution of misalignment of a hole of 0.2 mm, machined by the first spindle, with respect to the designed center. Bcx and Bcy represent an average value of misalignment of the center of the exposure master hole, machined by a second spindle different from the first spindle described above, with respect to the designed center and Bmx and Bmy represent a center value of distribution of misalignment of a hole of 0.2 mm, machined by the second spindle, with respect to the designed center. Radii ar and br of the distributions of misalignment are almost equal in cases of any spindle as long as the diameter is the same. However, the misalignment of center of the respective distributions with respect to the center of the exposure master hole is different, and the misalignment of the second spindle is larger than that of the first spindle. Accordingly, when a line width a pattern exposed based on the exposure master hole is narrow, there arises a case when the hole of 0.2 mm machined by the second spindle deviates out of the pattern.